(1) Field of the Invention
This invention relates to a method to epitaxially grow a III-V nitride film, particularly AlxGayInzN (x+y+z=1) film on a given substrate using a Metal Organic Chemical Vapor Deposition (MOCVD) method and an apparatus for the same method.
(2) Related Art Statement
In opto-electronic devices such as light-emitting diodes, laser diodes or photodiodes, it is proposed that III-V nitride films having their compositions of AlxGayInzN(X+Y+Z=1) are epitaxially grown on a given substrate made of sapphire single crystal, for example. Up to now, the epitaxial growth of the AlxGayInzN film has been performed using a MOCVD method or recently, a Hydride Vapor Phase Epitaxy (HVPE) method.
In the case of making a GaN film using a HVPE method, first of all, a substrate made of sapphire single crystal is set into a reactor in which a gallium metallic material is charged. Then, a hydrochloric acid gas is introduced into the reactor and reacted with the gallium metallic material, to generate a hydrochloric gallium gas. Then, an ammonia gas is introduced into the reactor and reacted with the hydrochloric gallium gas, to deposit and fabricate the GaN film on the substrate. The HVPE method has a higher film growth rate than a MOCVD method or a MOVPE method. For example, in the MOVPE method, a GaN film can be epitaxially grown typically at only several μm/hour, but in the HVPE method, the GaN film can be epitaxially grown typically at several hundreds μm/hour. Therefore, the HVPE method has its advantage in forming a thicker III-V nitride film.
However, a good quality AlxGayInzN film can not be provided by the HVPE method, and the fluctuation in thickness on the same substrate may be increased. On the other hand, forming the AlxGayInzN film via the MOVPE method is a time consuming process, and thus, the fabrication cost of the AlxGayInzN film is relatively high.
In the case of making an AlxGayInzN (x+y+z=1) film using a MOCVD method, a given substrate is set and held on a susceptor installed in a reactor, and is heated to a predetermined temperature by a heater. Then, a trimethylaluminum gas, a trimethylgallium gas, a trimethylindium gas or the like as III raw material gases are introduced with a carrier gas composed of a hydrogen gas or a nitrogen gas into the reactor. An ammonia gas as a V raw material gas is introduced with a carrier gas composed of a hydrogen gas or a nitrogen gas into the reactor. Then, the III raw material gases and the V raw material gas are reacted, to deposit and form the AlxGayInzN film on the substrate. As the AlxGayInzN film, an aluminum nitride film, a gallium nitride film, an indium nitride film, an aluminum-gallium nitride film, an aluminum-indium nitride film and a gallium-indium nitride film are exemplified.
In the above conventional method such as a MOCVD method, if the reaction between the III raw material gases and the V raw material gas is created on the wall surfaces of the reactor, the film-forming efficiency is degraded, and thus, the film growth rate is decreased. In the past, therefore, although it is considered that a cooling jacket is attached to the reactor, it is very difficult to directly attach the cooling jacket to the reactor because the reactor is made of quartz into a complicated figuration. As a result, the reactor is covered with a stainless tube on which a cooling jacket is attached.
In such a conventional fabricating apparatus, since the reactor is only indirectly cooled by the cooling jacket, it can not be cooled down sufficiently. Particularly, it was confirmed that an AlN film or an Al-rich AlxGayInzN (x+y+z=1, x>0.5, y≧0, Z≧0) film can not be fabricated sufficiently, as compared with a GaN film or an Al-poor AlxGayInzN (x+y+z=1, 0≦x<0.5, y≧0, Z≧0) film. The reason is because in fabricating such an Al-rich AlxGayInzN film, a large amount of trimethyl-aluminum and a large amount of ammonia are employed as raw material gases and thus, the large proportions of the raw material gases are reacted on the wall of the reactor and on the susceptor heated to a higher temperature. As a result, the epitaxial growth of the Al-rich AlxGayInzN (x+y+z=1, x>0.5, y≧0, Z≧0) film is inhibited.
Particularly, in the case that the susceptor is set on the bottom wall of the reactor and the substrate is set on the susceptor so that the main surface of the substrate is faced to the top wall of the reactor, a large amount of aluminum nitrides may be deposited on the top wall because the top wall is easy to be heated to a high temperature due to the radiant heat from the susceptor. Since the aluminum nitrides may be broken away from on the top wall and introduced into the growing Al-rich AlxGayInzN film, the crystal quality of the Al-rich AlxGayInzN film may be deteriorated.
In light of the above-mentioned problems, such a technique as to cool down the raw material gases with the cooling jackets attached to the nozzles to introduce the raw material gases into the reactor is disclosed in the Japanese Laid-open Publications Kokai Hei 10-167883 (JPA 10-167883) and Kokai Hei 10-67884 (JPA 10-67884). Moreover, such a technique as to cool down the raw material gases around the susceptor with a cooling jacket provided on the upper side from the susceptor is disclosed in the Japanese Laid-open Publication Kokai Hei 10-100726 (JP A 10-100726).
According to such a conventional technique, although the film growth rate and the crystal quality of the Al-rich AlxGayInzN film can be improved to some degree, they are not sufficient. Particularly, since the portion of the top wall of the reactor opposing to the substrate on the susceptor is not cooled down sufficiently, the broken away aluminum nitrides may deteriorate the crystal quality of the Al-rich AlxGayInzN film to large degree.
In addition, when using the conventional technique, the fluctuation in thickness of the AlxGayInzN film is increased. Particularly, when employing a larger substrate such as a 3-inch wafer, the fluctuation in thickness becomes conspicuous.
Moreover, it is proposed that a vertical reactor tube is employed and a substrate is set vertically in the reactor tube, that is, substantially parallel to the wall of the reactor tube. In this case, although raw material gases are introduced into the reactor tube from the top via nozzles cooled down with a cooling jacket, the nozzles may be stopped up through the reaction between the raw material gases in the nozzles.